Caption: Polarising filters. Two light-polarising filters overlapping. In normal white light the vibrations in the light waves are perpendicular to the direc- tion of flow but can point in any direction through 360 degrees. In polarised light the vibra- tions are confined to a specific plane. These two filters are rotated so that they each transmit light polarised in a different plane. As a result, no light can pass through both and the overlap appears dark. If one filter was rotated through 90 degrees, however, the planes would line up and the overlap would appear white. See photograph A200/ 322 for the same filters after rotation of handle through 90 degrees.